To produce a thermoelectric output, two materials 2,3 with dissimilar thermoelectric properties are joined so that, when the junction 1 of the two materials is heated or cooled, a voltage may be measured across the two materials. This is illustrated schematically in FIG. 1. A thermocouple therefore consists of two separate materials, which are often referred to as the positive 2 and negative 3 legs. A common requirement for these materials is that they are in the form of wire or thin strip, in order to achieve the required thermoelectric output whilst using the minimum amount of material.
There are a number of standard thermoelectric materials that are commonly used as legs of standard types of thermocouples. The standard types, for example K-type or N-type, have well defined thermoelectric responses, and other properties such as corrosion resistance are also well understood. To produce thermocouples conforming to the standard types, which are defined by national and international specifications, alloys of typical compositions are selected for each leg. There is a natural variation in the composition and properties of alloys used to form these legs, and so, where higher accuracy thermocouples are required, several combinations may have to be considered before a suitable match is found. This requires a thermocouple manufacturer to have several batches or lots of each standard material available, to ensure a satisfactory selection can be achieved.
When selecting a thermocouple, a user will typically choose a standard thermocouple that most satisfactorily fulfils the user's required condition. Conditions may be directly related to the thermoelectric response, for example a user may require a thermocouple that will provide a certain thermoelectric response over a specified temperature range. Other conditions such as chemical resistance in a particular environment may also apply. Hence, in order to meet a required specification, a user typically forms a standard type of thermocouple by selecting positive and negative legs from a stock of alloy material.
Sometimes a user may require a thermocouple having a thermoelectric response that does not conveniently fall within the specification of a standard type of thermocouple, or within the specification of any combination of known thermoelectric materials. In some applications, for example, the thermocouple output may be required to be positive by a few degrees, placing it outside the normal tolerance band of standard types of thermocouples formed by typical manufacturing processes. Alternatively, a user may require a thermocouple to achieve a specific accuracy, but there may be a limited source of material available for selection of the thermocouple legs. Thus, there may not be suitable material available to form a thermocouple having the required output. In such cases it may be desirable to have a process which allows fine tuning or tailoring of the thermoelectric output of a thermocouple.